Motion sensors and distance sensors have many applications (door openers, automatic conveyor line control, velocity measuring devices, collision avoidance devices, etc.); but their application in intrusion alarms, alias burglar alarms for securing an interior area, is probably the best known. One class of motion sensors emits radiation of a (hopefully) well-controlled and predetermined frequency (.omega..sub.O), which is reflected by an object in motion in the secured area and returns to the sensor as a (weak) signal with Doppler-shifted frequency (.omega..sub.O .+-..omega..sub.DS), the Doppler-shift being a measure of the component of the moving object's vector velocity in the direction of the sensor.
All motion sensors and distance sensors utilizing microwave or other similar radiation rely initially upon generation of radiation of the predetermined frequency in an appropriate cavity and emission of this radiation from an adjacent, related structure. Generally, the cavity used for generation supports both the frequency of interest and several higher harmonics; and the apparatus may emit these higher harmonics as well, often violating the Federal Communication Commission standards for the permissible frequency range in which motion sensors may be operated. Conventional motion sensors, if they address this higher harmonic problem at all, usually include an elaborate addon device to suppress, with indifferent success, the radiation modes corresponding to these unwanted higher harmonics.
Further, in most conventional motion sensors and distance sensors, separate but adjacent mixer apparatus must be provided for sensing the return signal with its Doppler-shifted frequency and for comparing the return frequency with the emitted or reference frequency; that is, the mixer appratus and the reference frequency generator and emitter apparatus are not truly integrated.